The Brucejack property lies within the Hazelton group, an Early to Mid-Jurassic volcano-sedimentary sequence of rocks formed during the formation of the Stikina island arc. The Hazelton group and the underlying rocks of Stikinia host several important economic mineral deposits relating to arc volcanism including; porphyry, epithermal, and volcanogenic massive sulphide deposits. The Brucejack deposit is a transitional epithermal gold silver occurrence hosted in stockwork veining located up stratigraphy from several large porphyrytic intrusions.
Submarine andesitic volcanic flows, breccias, tuffs, and associated immature sediments underlie much of the Brucejack Property. This is interpreted as intra arc basin fill in which numerous facies changes, growth faults, and stratigraphic boundaries acted as low porosity conduits for fluid flow. Stockwork veining occurred as result of a telescoping porphyry system introducing magmatic fluids through these conduits to the overlying strata. Depressurization and chemical reactions with host rocks and sea water is thought to have initiated the precipitation of gold and silver bearing quartz-carbonate veins. As a result of this fluid flow, the host rocks display intense quartz-sercite-pyrite alteration along a broad band that loosely follows a stratigraphic contact between an underlying layer of conglomerate and overlying andesitic fragmental rocks. All of the mineralized zones on the Brucejack property are located within or close to this alteration band.
Gold and silver mineralization occurs as coarse electrum in several generations of quartz-carbonate veins and vein breccias. Sulphide mineralization is present in most of the veins with pyrite, sphalerite, galena, chalcopyrite, and pyargerite being most commonly present.
The bulk of the gold resource at the Brucejack property is located within the Valley of the Kings. Here, coarse electrum mineralization occurs within close proximity to the conglomerate-andesite fragmental contact that outlines an eastward plunging syncline. Along this contact, most predominantly on the southern limb, a layer with intense silica alteration has formed. It is thought that this layer has acted as an impermeable boundary allowing pressure to build up below. Subsequent breaking or fracturing or breaking of this boundary may have resulted in rapid depressurization, boiling and deposition of electrum in very high grade veins yielding drill core assays up to 41.5 kg/t gold.